Method for assembling electrolytic capacitor and heat sink

ABSTRACT

An electrolytic capacitor with several conductive layer portions, including some anode layers and some cathode layers in alternating arrangement. A set of insulator layers is interleaved with the conductive layers, and the conductive and insulator layers are laminarly stacked with an anode layer being outermost on one surface. The stack is positioned adjacent a metal heat sink, with an outer insulator layer positioned between and closely contacting the stack and the heat sink. The cathode layers may be connected to the heat sink.

This is a divisional of application Ser. No. 08/978,212, filed on Nov.25, 1997 now U.S. Pat. No. 5,894,402.

FIELD OF THE INVENTION

The invention relates to capacitors, and more particularly to capacitorsfor applications requiring substantial heat dissipation capabilities.

BACKGROUND AND SUMMARY OF THE INVENTION

Like many electronic components, capacitors generate heat under certainconditions, and this heat must be controlled and dissipated to avoidcomponent and system damage. Capacitors in particular generatesubstantial heat when subjected to high ripple currents. When suchcurrents must be tolerated, capacitors must be made robust, requiringthem to be larger than would otherwise be desirable. Whereminiaturization is particularly critical, designers face an unwelcometradeoff between size and current handling capacity.

To reduce these disadvantages, heat generating components have beenfitted with radiative heat sinks in contact with their external housingsurfaces. Such heat sinks may also be in the form of a metal circuitsubstrate to which the components are mounted. A limitation ofconventional heat sinks is that close thermal coupling is difficult.Adhesives used to maintain a large contact area must generally beelectrically insulative which limits their thermal conductivity.Conventional component housings also increase the thermal isolation ofthe hot conductors in the component and the heat sink. In addition, itis often desirable to provide capacitors with the maximum capacitanceper unit volume, and current designs are limited in this respect.

The present invention overcomes the limitations of the prior art byproviding an electrolytic capacitor with several conductive layerportions, including some anode layers and some cathode layers inalternating arrangement. A set of insulator layers is interleaved withthe conductive layers, and the conductive and insulator layers arelaminarly stacked with an anode layer being outermost on one surface.The stack is positioned adjacent a metal heat sink, with an outerinsulator layer positioned between and closely contacting the stack andthe heat sink. The cathode layers may be connected to the heat sink.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a heat sink-mounted flat capacitoraccording to a first embodiment of the present invention.

FIGS. 2 is an enlarged sectional view of the capacitor of FIG. 1.

FIG. 3 is an exploded view of the components of a rolled capacitoraccording to an alternative embodiment of the invention.

FIG. 4 is a sectional side view of a heat sink-mounted rolled capacitorthe embodiment of FIG. 3.

FIG. 5 is an enlarged sectional side view of the capacitor of FIG. 3.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a capacitor assembly 10 including a flat capacitorstack 12 mounted within a rectangular recess 14 of an aluminum circuitsubstrate 16. The capacitor stack, shown somewhat schematically withoutinsulative layers, is a stack of alternating cathode layers 20 and anodelayers 22. The anode layers are electrically connected to each other andto circuitry on the substrate or elsewhere, as are the cathode layers. Alowest anode layer 23 is positioned closest to the floor of the recess,and the top conductive layer is a cathode sheet 20.

As shown in FIG. 2, insulating separator sheets 24 are distributed amongthe conductive anode and cathode sheets, with one separator between eachconductive pair to prevent electrical contact and to maintain acontrolled small gap. A lower separator sheet is positioned between thelowest anode layer 23 and the floor of the substrate recess 14 toprevent electrical contact while offering minimal thermal resistance.The anode sheets are undersized relative to the recess so that theiredges do not make electrical contact with the substrate, and theseparator sheets extend beyond the peripheries of any of the conductivesheets to prevent electrical shorting at any edge.

Referring back to FIG. 1, the substrate has an electrically insulativeupper film 30, and a pattern of conductive traces 32 applied to thefilm, and connecting to other electronic components (not shown). Theanode sheets 22 of the capacitor are electrically interconnected to eachother, and to an anode contact trace 32, or to any other circuitcomponent as may be needed for a given circuit design. The cathodesheets 20 are interconnected to each other and to the substrate materialitself, causing the substrate to function as an additional cathodelayer. An epoxy encapsulant 34 may be applied over the capacitor stackto fill the recess, to enhance thermal conduction between the stackedges and the substrate, and to provide an environmental seal and ruggedprotection for the capacitor layers. In alternative embodiments, anadhesive film may be used to seal the components while maintaining a lowprofile. In other embodiments where heat is a major concern, a metalheat sink may be adhered atop the film or encapsulant. Also, the chamberdefined by the recess and cover may be filed with an electrolyte.

The interaction of the lowest anode layer 23 with the substrate, asspaced apart only by the lowest separator layer 26, provides additionalcapacitance without using an additional cathode layer. In addition, asthe separation between the lowest anode layer 23 and the substrate isextremely small, thermal conductivity is greatly enhanced. With thelowest separator layer having an area of 20 cm², and a thickness of0.015 cm, a high thermal conductivity geometry coefficient (area dividedby thickness) of 1300 cm is provided.

Although not illustrated, the connections between layers may be made bymeans conventional to flat capacitors. In a preferred embodiment, a setof cathode tabs registered with each other extends beyond the peripheryof the separator layers, as disclosed in U.S. Pat. No. 5,522,851 toFayram, which is incorporated herein by reference. The cathode tabs arethen welded or staked together, and connected to desired circuitry. Thethicker and more rigid anodes may be provided with tabs and may bewelded together before installation in the substrate, with the cathodesdefining a cut out near the anode tabs to provide clearance. An aluminumfoil conductor may be welded to the anode tab edges, and welded orstaked to the circuit as shown. In the preferred embodiment, the anodelayer is a 0.004 inch thick sheet of deeply etched ultra pure aluminumwith a grain structure that is perpendicular to the surface of thesheet. Each separator is a paper sheet is 0.002 inch thick, and eachcathode sheet is 0.0006 inch thick sheet of pure aluminum. With atypical separator sheet providing minimal spacing of less than 10microns between the closest anode layer and the heat sink, close thermalcoupling is provided.

In a preferred example, each sheet has a width of 5 cm and a length of 4cm. About 10-30 pairs of anode and cathode sheets are stacked, and atotal capacitance of 400 μF is provided. The substrate may also beprovided with fins to improve heat dissipation. The substrate functionsas a heat sink and an electrical conductor. However, in alternativeembodiments in which electrical conduction by the substrate is notdesired, an electrically insulative and thermally conductive heat sinkmaterial such as ceramic or porcelainized steel may be used.

In FIGS. 3, 4, and 5, a rolled cylindrical capacitor according to analternative embodiment is illustrated. As shown in FIG. 3, a stack 100of elongated rectangular sheets is prepared, then rolled into theconfiguration shown in FIGS. 4 and 5. The top sheet is a cathode layer102, with a first separator 104 below, then an anode sheet 106, and alower separator layer 108. The sheets are closely stacked to overlayeach other, with the separators extending peripherally beyond theconductive sheets as in the preferred embodiment. The stack is describedas having an inner end 112 at the left of the illustration, and an outerend 114 at the right. At the outer ends of the cathode and anode sheets,respective tabs 116 and 120 extend from positions offset from eachother.

As shown in FIG. 4, the stack is tightly rolled about a core or mandrelinto a cylinder 122. The inner end 112 is nearest the center of thecylinder, while the outer end remains at the surface, with tabs exposed.The stack has been rolled with the upper surface of the cathode curvedinto a concave shape, with the lower separator layer 108 forming theoutermost layer of the roll, and the anode forming the next layer belowthe outer layer.

A metal substrate 124 defines a semi-cylindrical recess 126 having aradius substantially the same as the outside radius of the roll 122, sothat the roll may be closely received within the recess for maximumsurface contact. As better shown in FIG. 5, the cathode tab 116 iselectrically connected to the substrate, adding to the capacitance ofthe device as in the preferred embodiment, and the anode tab 120 isconnected to circuitry 32 that is electrically isolated from thesubstrate. The roll is encapsulated by an epoxy layer 130, or may becovered by a semi-cylindrical lid or heat sink. If a fully recessedconfiguration is desired, the recess may be deepened, with straight sidewalls extending upward to the surface from the semi-cylindrical portion.

One contemplated example of the cylindrical embodiment has elongatedsheets of 2 cm width, 50 cm length, and which are rolled to a diameterof 1.5 cm, providing essentially 15 anode-cathode layer pairs from thecore to the periphery of the roll. In ideal circumstances usingoptimally etched anodes, this is expected to provide a capacitance of400 μF, and to be capable of withstanding ripple currents specified as25% of rated DC voltage, with contemplated DC voltages of 200-400V.Alternative embodiments are also contemplated for the rolled capacitorbased on the variants discussed above with respect to the flatcapacitor.

In the disclosed embodiments, the capacitor stack, whether rolled orflat is substantially smaller in area than the surface of the substrateon which it rests. The longest dimension of the substrate issubstantially larger than the longest dimension of the stack, so that asignificant portion of the substrate extends beyond the stack toeffectively radiate generated heat. In the preferred embodiments, thesurface area of the upper surface of the substrate not covered by thestack is greater than the area covered by the stack, and substantiallyso. The exposed rear surface of the substrate further enhances theradiating area.

While the invention is described in terms of a preferred embodiment, thefollowing claims are not intended to be so limited.

What is claimed is:
 1. A method of assembling an electrolytic capacitorcomprising the steps:providing a rigid metal substrate having an overallarea and an exposed metal capacitor attachment region having asubstantially smaller area than the overall area; assembling a stack ofalternating anode and cathode layers; assembling the stack includingpositioning an insulating separator sheet between each of the anode andcathode layers; assembling the stack including positioning a lowestanode layer at a lower surface of the stack; positioning a finalseparator layer below the lowest anode layer; and positioning the stackinto contact with the capacitor region of the substrate, such that thelowest anode layer is separated from the substrate only by the finalseparator layer.
 2. The method of claim 1 including electricallyconnecting at least one of the cathode layers to the substrate.
 3. Themethod of claim 1 including defining a cavity in the substrate, andwherein positioning the stack into contact with the capacitor regionincludes positioning at least a portion of the stack within the cavity.4. A method of assembling an electrolytic capacitor comprising thesteps:providing a rigid metal substrate having an overall area and anexposed metal capacitor attachment region having a substantially smallerarea than the overall area; providing an elongated lower separatorlayer; positioning a conductive anode layer on the lower separator layerpositioning an intermediate separator layer on the anode layer;positioning a conductive cathode layer on the intermediate separatorlayer; rolling the layers into a cylinder with the lower separator layeris the outermost layer of the cylinder; and positioning the cylinderagainst the capacitor attachment region of the substrate.
 5. The methodof claim 4 including electrically connecting the cathode layer to thesubstrate.
 6. The method of claim 4 including compressing the cylinderagainst the substrate.
 7. The method of claim 4 including defining acavity in the substrate, and wherein positioning the cylinder againstthe capacitor region includes positioning at least a portion of thecylinder within the cavity.